Is there fuel savings with deep setback with steam heat?

ChrisJ

Steam systems aside setbacks no matter how small must save something.

The smaller the difference between indoor and outdoor temperatures means less heatloss

The claim of having to reheat objects in the house wasting energy is false as energy cannot be destroyed. When the room cools back down you get 100% of what you put into all of those items back out. This causes the room's temperature to fall slower, but who cares.

I just did a 4 degree recovery this morning with a 12F ambient in less than 40 minutes and that's with an allegedly undersized boiler. I can do a 12 degree recovery with a 12F ambient in around 1.5 hours.

Gordy

But how long would the system have ran with out set back? How much longer did it run during recovery verses if it were just maintaining that set point?

Gordy

In your 12* set back example would the boiler have ran 1.5 hours holding the regular set point in that setback period anyway? If not what would it have ran.

ChrisJ

Gordy said:

But how long would the system have ran with out set back? How much longer did it run during recovery verses if it were just maintaining that set point?

I don't know, but what I am sure of is it ran less.

Captain Who

"But how long would the system have ran with out set back?"

That's hard to say because I am uncertain how much to weigh the effects of solar radiation, but the above data summary answers your question. It would have run more than 8.9% longer. The effect of solar radiation is far from insignificant and it may be that if you knew how to factor it (I'm working on it ), it may have run as much as 15% longer. During my testing period the Solar Radiation for the Normal Operation Hours vs Setback+Recovery Hours was 134.4 Watts per sq meter vs 9.2 Watts per sq meter, from the downloaded weatherunderground.com data.

Gordy

Maybe, but how much less? Your MRT certainly lagged your setpoint once recovery was complete I know that.

ChrisJ

Gordy said:

Maybe, but how much less? Your MRT certainly lagged your setpoint once recovery was complete I know that.

What's MRT stand for?

Gordy

So in Chris's example of 1.5 hours he would have had an additional run time of 8.55 min. If he has a 200K boiler that pans out to 3333.33 btu input per minute. So he would have used 28333.33 btus more. If on NG at current rate in my area thats .28 therms times .60 cents or 17 cents for .28 therms use. 5.10 for the month, or 30 bucks for a 6 month heating season.

Granted this is not your scenerio, and im throwing numbers around that probably do not fit, and savings depends on fuel type. But I think its negligable savings for the comfort lost if the sole goal is to reap seemingly large savings.

The end answer to the question is yes of course, and milage varies....

Gordy

ChrisJ said:

Gordy said:

Maybe, but how much less? Your MRT certainly lagged your setpoint once recovery was complete I know that.

What's MRT stand for?

The mean radiant temperature of the mass of your home.

Captain Who

When I get up around 7:30ish am I am SUPER comfortable because the ceilings are all warm and the radiators are blazing hot all the way across. The apparent comfort level relative to the normal operation period setpoint is achieved prior to the thermostat being fully satisfied on the recovery cycle. The second recovery cycle typically does not finish until somewhere betwen 8:45 and 9:00 am in my case but I am comfortable around 7:30 (excluding the temperature of my computer chair ).

I don't feel that it is negligible savings, provided it does not reduce the life of my boiler or radiator vents. That is an issue that is hard to quantify. My gas bill during the Nov thru Mar '14 period was only $480ish, so a 15% savings on that is only $72. If someone had a $2500 fuel bill that would be $375.

Gordy

Agreed. And the savings wil be a float all depending on outdoor temps, and structure to exterior deltas, solar etc. I think emitter, and system type adds to those variables. Comfort to me is neither feeling hot, or cold. actually your not aware of anything affecting it.

ChrisJ

Gordy said:

ChrisJ said:

Gordy said:

Maybe, but how much less? Your MRT certainly lagged your setpoint once recovery was complete I know that.

What's MRT stand for?

The mean radiant temperature of the mass of your home.

I agree, it lags the setpoint.

ChrisJ

Gordy said:

Agreed. And the savings wil be a float all depending on outdoor temps, and structure to exterior deltas, solar etc. I think emitter, and system type adds to those variables. Comfort to me is neither feeling hot, or cold. actually your not aware of anything affecting it.

True,
But this is also why people do it at night. Because you're in bed under a thick comforter completely happy.

Also, cooler air means higher RH and better breathing while you're asleep so even if you only save 20 cents a month it's still beneficial.

Let's also not forget about wear and tear on ignition and damper components. By doing a setback and recovery this is lowered as well.

PMJ

I agree with Chris that the bottom line is that the heat loss of the structure ultimately has only to do with the difference between the inside and outside temperatures. Anything setup that results in a lower average temperature difference will require less total heating input. All other factors (sun,wind) average out to be the same in a big enough sample and have no impact on setback savings.

And for the same reason there is no economic advantage to trying to minimize the recovery time unless a few degrees low is intolerable. Stretching out the recovery does in fact increase the time spent at lower inside temperature and therefore it lowers the average temperature difference too. I have found that with gently warm radiators all morning long and a 6 hour recovery no one notices the actual room temp being a degree or two under the usual for part of the way back up. I intentionally spread out the recovery.

Captain Who

"I agree with Chris that the bottom line is that the heat loss of the structure ultimately has only to do with the difference between the inside and outside temperatures."

Solar radiation actually has a big effect upon it. There is very little solar radiation during the overnight hours. In order to know how to mathematically model it I need to accumulate enough data so that I have enough regular operation period days with similar degree hours. Then I can graph the fuel usage during that period vs solar radiation and see if I can come up with an equation.

Wind has a tremendous effect upon it too. The convective heat transfer coefficient increases greatly with windspeed, for a given temperature difference. I factored that in by computing the wind chill temps and using that for my degree hours calcs.

EDIT To Add:

For example, yesterday was a dark day and for the Regular Operation Period vs Setback+Recovery Period the solar radiation was 10.71 vs 0.52. That gave a fuel usage of 0.762 cu ft per degree hr per hr vs 0.570 cu ft per degree hr per hr. That is a savings of 25.2% during the recovery+setback period, or a savings of 11.3% (vs 8.9% for my overall study) for the entire day. That's why I think solar radiation is clearly significant.

Gordy

Yes solar, and wind have great effect. But usually setbacks are night time, and that takes solar out of the equation, and wind usually is calmer at night. Now day time setbacks it's a whole different ball game.

ChrisJ

Captain Who said:

"I agree with Chris that the bottom line is that the heat loss of the structure ultimately has only to do with the difference between the inside and outside temperatures."

Solar radiation actually has a big effect upon it. There is very little solar radiation during the overnight hours. In order to know how to mathematically model it I need to accumulate enough data so that I have enough regular operation period days with similar degree hours. Then I can graph the fuel usage during that period vs solar radiation and see if I can come up with an equation.

Wind has a tremendous effect upon it too. The convective heat transfer coefficient increases greatly with windspeed, for a given temperature difference. I factored that in by computing the wind chill temps and using that for my degree hours calcs.

I can agree to this.
When I swapped my burner and drafthood it was in the low teens when I started and high teens when I finished,might have been an even 20F out but no warmer than that.

After several hours the house dropped from 72F to 62F, all but one room. A large bedroom on the second floor, which has a lot of exposure to sun most of the day stayed warm enough the TRV didn't even let the radiator heat.

Sun has a huge effect on things. It's a second reason I want to put TRVs in my kitchen, that and the oven.

Gordy

What all this means is really knowing your envelope, and heating system. The loser the envelope the more reward. The more mass the emitters have, and how much they depend on convection is another. Radiant is my camp so there is a huge difference.

If you have two identical water heaters set to the same temp. One gets turned off for x amount of time, and then put into recovery. The other just maintains set point I'll bet uses less fuel.

The tank is the envelope, the water is the mass, the burner the heat plant. That as simple as it gets.

Captain Who

"But usually setbacks are night time, and that takes solar out of the equation,"

But I would argue quite the contrary. If the sun did not shine at all during regular operation periods as it mostly does not during setback+recovery period, your actual % savings on your fuel bill would be less when doing overnight setbacks, but your overall fuel bill would be much larger too of course.

ChrisJ

Gordy said:

What all this means is really knowing your envelope, and heating system. The loser the envelope the more reward. The more mass the emitters have, and how much they depend on convection is another. Radiant is my camp so there is a huge difference.

If you have two identical water heaters set to the same temp. One gets turned off for x amount of time, and then put into recovery. The other just maintains set point I'll bet uses less fuel.

The tank is the envelope, the water is the mass, the burner the heat plant. That as simple as it gets.

I'm confused?
If one drops in temperature any more than the other, it must loose less energy.

Please explain.

Gordy

The water heater that is just holding temp will use less fuel than the one that loses temp during its setback, and has to recover to the original temp.

Captain Who

It depends upon how long the setback is for, and how much of a setback. There is a breakeven point. I know I didn't post my raw data because it is too much work, but don't you think my study proves that setbacks can save fuel? If not, why?

ChrisJ

Gordy said:

The water heater that is just holding temp will use less fuel than the one that loses temp during its setback, and has to recover to the original temp.

I'm going to respectfully disagree.

The water is heated by energy you put into it. By maintaining a higher temperature you increase your loss through the insulation.

Captain Who

We're getting off track from a steam heating system here, but if you had a laboratory grade water heater with imperfect insulation with a submerged electrical heating element and homogeneous water temperature by way of mechanical stirrers even the slightest setback would result in an energy savings. Otherwise, I think you are defying the laws of physics.

EDIT to add: where the water temp stabilizes at the lower setpoint temperature for any amount of time greater than zero, due to the lower heat loss rate. If not, the energy savings would probably be zero, but definitely not an energy loss.

ChrisJ

Captain Who said:

We're getting off track from a steam heating system here, but if you had a laboratory grade water heater with imperfect insulation with a submerged electrical heating element and homogeneous water temperature by way of mechanical stirrers even the slightest setback would result in an energy savings. Otherwise, I think you are defying the laws of physics.

I agree.

PMJ

To all the comments on sun wind etc. - if I must try to compare one day against one other day then yes these can change the results a lot. But I think such daily comparisons are pointless. The larger the number of days(years) in the sample, the less these things have anything to do with it. Daily changes in wind and sun are inevitable and not controllable. They do not have anything to do with whether setbacks on net increase or decrease overall efficiency in the long run.

Jamie Hall

You should save energy with a 12 degree setback, since you are effectively running your building 6 degrees colder. Whether you save that much is another question.

However. May I point out that in some houses, there may be additional costs which you have not factored in. For instance, the fastest way to ruin a piano is to have setbacks. If it's a cheap Wurlitzer spinet, which serves to hold up the flower arrangements and the cocktails, fine. If it's a Steinway concert grand, you don't... you really don't want to do that. It also affects genuine plaster. If you don't mind replacing the plaster every decade or so, fine. It doesn't bother plasterboard, though... and so on.

Captain Who

@PMJ - that kind of long term study is all well and good but what if you want to get some results in a short period of time, so you can find out the truth right now? I did a 2 week study and it was totally definitive because I separated the Setback+Recovery period from the Normal Operation Period, within each day and averaged these data over 11+ days.

Anyone who is curious about the feasibility of saving fuel through setbacks with their house and steam system could easily do the same thing.

PMJ

Jamie Hall said:

You should save energy with a 12 degree setback, since you are effectively running your building 6 degrees colder. Whether you save that much is another question.

However. May I point out that in some houses, there may be additional costs which you have not factored in. For instance, the fastest way to ruin a piano is to have setbacks. If it's a cheap Wurlitzer spinet, which serves to hold up the flower arrangements and the cocktails, fine. If it's a Steinway concert grand, you don't... you really don't want to do that. It also affects genuine plaster. If you don't mind replacing the plaster every decade or so, fine. It doesn't bother plasterboard, though... and so on.

I only use 3 degrees - just so the rads aren't hot at going to bed time. On an average day (25-35F) my house barely falls the 3 degrees from from midnight to 5 am anyway and it is 1926 with no insulation. So I don't know what people are talking about with 12 degree setbacks.

I hear what you are saying about preserving things - but I guess I just don't think things are so fragile as all that. I have a Steinway Grand and fine woodwork and plaster crown moldings all of which are much older than air conditioning. They seemed to have managed to survive all that without cracks even through the open window sleeping era etc. But I live here to enjoy life and I'm not trying to run a museum.

Bottom line is anything more than just a few single digit degree setbacks are not really possible anyway - or if your house falls faster than that in 6 hours on the average day without the windows open something else is really wrong.

I'm convinced what I do costs less than straight heat 24 hours but cost is not the reason I do it.

PMJ

Captain Who said:

@PMJ - that kind of long term study is all well and good but what if you want to get some results in a short period of time, so you can find out the truth right now? I did a 2 week study and it was totally definitive because I separated the Setback+Recovery period from the Normal Operation Period, within each day and averaged these data over 11+ days.

Anyone who is curious about the feasibility of saving fuel through setbacks with their house and steam system could easily do the same thing.

I don't disagree with what you are saying. It's just that I am satisfied that setbacks are more efficient just by the temperature difference logic. Exactly how much is out of my control so I will just take whatever I get. I don't think I could accurately account for the individual day differences of other variables enough to make the comparison you are attempting. More power to you if you can.

jch1

Jamie Hall said:

You should save energy with a 12 degree setback, since you are effectively running your building 6 degrees colder. Whether you save that much is another question.

However. May I point out that in some houses, there may be additional costs which you have not factored in. For instance, the fastest way to ruin a piano is to have setbacks. If it's a cheap Wurlitzer spinet, which serves to hold up the flower arrangements and the cocktails, fine. If it's a Steinway concert grand, you don't... you really don't want to do that. It also affects genuine plaster. If you don't mind replacing the plaster every decade or so, fine. It doesn't bother plasterboard, though... and so on.

What kind of setback would have an effect on plaster walls? I didn't realize that was an issue with setbacks.

Gordy

Any building has stress form delta t's inside verses outside temps.

Think of a wall assembly, and the temperature difference from outer skin to inner skin. Some where in the middle is the mason Dixon line. Winter time is the worst because of the huge delta of set point and outside temps verses summer.

You have an outer contracting skin, and an expanding inner skin in the winter.

In the summer it's the opposite, and less stress.

Winter deltas could easily reach a delta of 75. Where summer is more like 20.

Around my area have seen -27 everybody's house was popping in the middle of the night.

With plaster,temperature consistancy gives longevity. Same holds true with all other things like Jamie pointed out.

vaporvac

And humidity doesn't help. That was the killer for me. However I was careful bringing my house back up from a 40deg. set-back very slowly. I suspect the main reason nothing was damaged is it took so long to actually bring EVERYTHING up to temp.

Jamie Hall

Thanks Gordy! And I would add, before folks get too confused, that the real problem with plaster... pianos... fine furniture... and such like items is not so much either the absolute temperature and humidity, but the speed with which it changes. For example. Consider a grand piano. In the place I care for, there are two; one is in a building which isn't heated (a summer "artist's studio") and the other is in the main building. Neither one has been hurt by temperature change -- the one in the unheated building, because temperature change is slow. The one in the main building because temperature change doesn't happen. But suppose the temperature does change quickly -- as in a large (or according to EPA, reasonable!) setback. The iron frame responds to the temperature change quickly. The sounding board does not. The sounding board cracks (if not the first time, after the first say ten or twenty fast swings). You have just cost yourself several tens of thousands of dollars. A concert grand actually used as such should be tuned after every temperature change of more than a few -- three or four -- degrees. You can buy a lot of oil or gas for one good tuning.

The same sorts of arguments apply to a lot of things. Most folks don't have to worry about them. Some folks do. One size does not fit all!

jch1

Well this is certainly interesting information. This is our first heating season with the house, and I've been using a 2-4 degree setback regularly. I've noticed there are some rather substantial cracks in the house at different locations, but the biggest one appears between the chimney (which is in the center of the house, not on an outside wall) and the adjacent wall. I wonder if the plaster walls are warming up at a different rate than the chimney, which is exhausting the gases via a stainless steel liner, and the plaster walls.

hmm

PMJ

I can easily agree that anything held at a constant temperature for its entire life will see less stress.

But before we panic too much about 2-4 degree temperature changes destroying our homes consider a couple things. Most churches, for instance, can't really afford to heat vast sanctuaries 24/7 to the 72 the parishioners desire for 6 hours on Sunday. Many around me, vast steam heated structures from the building boom of the 1920's with priceless pipe organs and plaster and woodwork inside really do use setbacks. One I am quite familiar with allowed the temperature to fall 10 degrees during the week and has done so for decades. It also still has no air conditioning.

And with regards to speed of temperature change, the good news here is that too fast really isn't possible going down or going up. The structure I referred to above would take easily more than 24 hours to drop 10 degrees and require 12 hours anyway to be brought back up. Frankly I don't believe those who report recoveries of many degrees in an hour or so. The heating system would have to be massively oversized to move the entire structure and all of its contents several degrees/hour. On design day the heating system has all it can do just to stay even - it couldn't move the whole structure up at all.

I think we are not doing jch1 any favors if he ends up worried such a small setback is cracking up his home.

ChrisJ

PMJ said:

And with regards to speed of temperature change, the good news here is that too fast really isn't possible going down or going up. The structure I referred to above would take easily more than 24 hours to drop 10 degrees and require 12 hours anyway to be brought back up. Frankly I don't believe those who report recoveries of many degrees in an hour or so. The heating system would have to be massively oversized to move the entire structure and all of its contents several degrees/hour. On design day the heating system has all it can do just to stay even - it couldn't move the whole structure up at all.

I think we are not doing jch1 any favors if he ends up worried such a small setback is cracking up his home.

Why thank you PMJ,

My radiation is 392SQFT and my home is 1616sqft with a heatloss of around 60,000.

That makes the system 34,080 btus oversized, not counting losses from the mains that ends up in the house as well. It was installed in the early to mid 20s when I'm told most systems were grossly oversized to allow for sleeping with windows open in the bedroom.

I can assure you my system has absolutely no problem doing a 6 degree recovery on a 0F design day in only 1 hour. After the system shuts down and the rooms all coast up to around the set point it will cause the temperature to drop a little faster than normal until things stabilize, but overall the place is comfortable and the air temperature is near setpoint.


Technically, design day in our area is about 4F but I used 0F when I did my heatloss. Our record low is -17F and we see below zero often enough.

Captain Who

"Frankly I don't believe those who report recoveries of many degrees in an hour or so. The heating system would have to be massively oversized to move the entire structure and all of its contents several degrees/hour. "

What don't you believe exactly? The thermostat only knows the air temperature and to some extent the temp of the wall it is mounted on. What takes a long time to go up also takes a long time to go down. The air is the thing that changes rapidly in temp, but the objects take longer to go down and longer to go up, due to their larger heat capacity and thermal mass, so that even though the air temp has stabilized at the setback temp, that might not and probably isn't the case that all the objects have gone that low, because not enough time may have passed.

My boiler is unfortunately oversized by 46% and it usually takes about 1hr and 40m for my house to recover from the overnight setback of 8 deg, depending upon a lot of factors, mostly outside temp and wind speed. That is going by the second cycle of the tstat, ie. not the first time it thinks it has recovered to the daytime setpoint. Do I have dataloggers monitoring the internal temps of every object in my house? Nope.

PS: I also have ceiling fans in the first floor rooms that are blowing upwards at low speed and that helps to even out the temps of the objects through convection.

PMJ

Chris,

The starting place I am talking about is everything 6 degrees down in your case - structure, contents, and boiler/piping. I don't know how fast you can get steam to the rads from your boiler at room temp but the number I hear most often is around 20 min. That leaves 40 min for your boiler to raise the entire structure and contents 6 degrees - and no I don't believe it - especially at 0F outside. And yes, I mean stabilized. So from your own account you have to give on the one hour already. Whether the period prior to stabilization is comfortable to you or not is a different subject.

I have a 462,000 btu input boiler connected to 1000EDR running at 1-2 oz or so. I think that is no less oversized than yours and I assure you I can't raise my structure and contents that fast even at 20F outside. But even more importantly - I am glad I can't as it would make everyday operation even more difficult to even out.

It seems really important to you that your system can do this Chris so I will leave you to it. My effort here was only to try to use a little common sense in the setback discussion when addressing potential damage to our homes.

Captain Who

So to put it in a constructive manner, the question at hand is how do you determine the point at which the house has really completed recovery? I'm willing to admit that the best way and most scientifically rigorous way would be to have temperature sensors in all the rooms at different heights and on all the walls, ceilings and floors and a few pieces of furniture as well and to average all of those readings together at the point where the thermostat last completes a cycle before setback, and to say that recovery has taken place when you have reattained that average.

But in my case it is just a rough first effort to answer the question of this thread and it is what it is. The results clearly reveal a significant fuel savings in my case. Not being able to factor in solar energy yet, and not knowing what the exact point of recovery is are areas where there is room for improvement.